Tin is resistant to corrosion and is used as a protective coating on less resistant metals, such as steel. One method of applying a tin coating is to dip a steel plate into molten tin. However, this method is wasteful because it typically produces a thicker layer of tin than is necessary. Consequently, electrolytic methods, which produce a thinner and more uniform layer of tin, have been developed. Electroplating of tin onto steel strip is disclosed, for example, in Kitayama, U.S. Pat. No. 4,181,580, the disclosure of which is incorporated herein by reference.
In the high speed tinning of strips of steel, the strips of steel are first cleaned in a series of alkaline cleaners to remove oils and greases. Then the steel passes through several water rinses and then into a dilute acid (“pickling”) solution before passing into the electrolyte plating bath, which produces a layer of tin on the steel surface. The layer of tin, as deposited, typically has a smooth matte surface.
Two tin plating solutions are commonly used in strip steel tin plating baths. The FERROSTAN® system contains phenolsulfonic acid (HOC6H4SO3H, PSA) and stannous sulfate, while the RONASTAN® system contains methanesulfonic acid (CH3SO3H, MSA) and stannous methanesulfonate. The use of MSA in electrolyte baths is disclosed, for example, In Thompson, U.S. Pat. No. 5,312,539, and in Copping, U.S. Pat. No. 6,251,255, the disclosures of which are incorporated herein by reference. The use of PSA acid electrolyte baths is disclosed, for example, in Ooniwa, U.S. Pat. No. 4,936,965, and in Dulcetti, U.S. Pat. No. 6,921,472, the disclosures of which are incorporated herein by reference.
After plating, the plated strip is typically rinsed twice with water. After rinsing, the plated strip then enters a fluxing solution (e.g., an “acid flux” solution), followed by air drying. The term “flux” refers to a substance that aids the reflow operation. The plated strip is then heated in a reflow oven to slightly above the melting point of tin (about 232° C.), typically in a reflow oven heated to about 240° C. The tin layer is melted, forming a surface layer of tin and a subsurface diffusion layer containing tin and tin-iron alloy on the steel substrate. After heating (“reflow”), the plated strip is rapidly cooled or quenched by immersion in water, producing a tin surface layer that has a bright finish.
The purpose of the rinse steps that follow plating is to remove as much of the components of the plating electrolyte solution from the tin surface as possible. Some of the plating electrolyte will be retained on the tin surface as “dragout” as it is removed from the plating bath. The dragout composition can include water, the plating acid (i.e., PSA or MSA), stannous salts, and dissolved electroplating additives. Because dragout of the components of the plating bath represents an economic loss, and because some water is lost from the plating bath due to evaporation or entrainment with gases evolved during the electroplating operation, the rinse solutions typically have a counter-current flow so that the rinse water and the plating bath components dragged into the rinse solutions with the plated strip are returned to the plating solution.
As discussed in O'Driscoll, U.S. Pat. No. 6,409,850, and in Allen, U.S. Pat. No. 2,719,820, the disclosures of which are incorporated herein by reference, the purpose of the fluxing agent is to remove oxide from the tin surface and to reduce the surface tension of the melting tin during reflow, thus preventing uneven flow of the tin during reflow. Such uneven flow can result in a non-uniform surface (e.g., “woodgrain”) after quenching. Examples of fluxing agents include hydrogen chloride, stannous chloride, zinc chloride, ammonium chloride, palm oil, gluconic acid, glutamic acid, citric acid, tartaric acid, citrazinic acid, chelidamic acid, chelidonic acid, cyclohexene-1,2-dicarboximide, various naptholdisulfonic acids, and various hydroxybenzenesulfonic acids, including PSA. Although PSA serves as a good fluxing agent, MSA is not suitable as a fluxing agent due to formation of blue stains, as discussed below.
When a FERROSTAN® plating solution, which contains PSA, is used, the concentration of PSA in the acid flux solution, due to dragin from the plating bath and the prior rinse, typically is about 0.1-1.0% of PSA. An acid flux solution that contains 0.1 to 1.0% of PSA produces a bright, adherent surface layer after reflow. However, because of the presence of free phenol in a plating solution that contains PSA and because PSA has a low inherent electrical conductivity, electrolytes other than PSA have been sought.
A plating solution that contains MSA is more worker friendly because it does not contain phenol and also more conductive than a plating solution that contains PSA. In addition, MSA is a non-oxidizing acid and minimizes the oxidation of stannous ion (Sn+2) to stannic ion (Sn+4). Stannic ion forms stannic sludge, an insoluble oxide sludge which precipitates from solution, resulting in a loss of tin from the electroplating system. When MSA is used in the plating solution, the acid flux solution contains MSA due to dragin from the plating bath. When MSA is present in the acid flux solution, after reflow the surface layer sometimes has an undesirable blue haze, which may be deleterious to the appearance of the tin surface and may also affect the corrosion resistance of the surface layer.
Thus, a need exists for tin plating processes that do not have the disadvantages of the process that uses PSA and yet does not lead to the formation of an undesirable blue haze after reflow.